Magnetic tachometer

ABSTRACT

A pair of toothed elements of magnetically permeable material are coaxially mounted for relative rotation with their teeth spaced opposed in rotational registration. The toothed elements are further mounted in magnetic circuit with magnets and a coil whereby magnetic flux provided by the magnets follows two separate magnetically parallel flux paths including the pair of toothed elements and the coil. As the toothed elements are relatively rotated, the net reluctance of the flux path through the tooth elements varies and causes a voltage to be induced in the coil which alternates at a frequency representative of the relative speed of rotation of the toothed elements. The other of the magnetically parallel flux paths is of a fixed reluctance and does not extend through the pair of toothed elements.

[ MAGNETIC TACHOMETER [75] Inventor: Alfred Hasler, Mountain View,Calif. [73] Assignee: Amper Corporation, Redwood City,

. Calif.

[22] Filed: Oct. 15, 1971 [21] Appl. No.: 189,685

[51] Int. Cl. H02k 19/24 [58] Field of Search 310/70, 168, 169,

[56] References Cited UNITED STATES PATENTS 3,504,538 4/1970 Andrews310/168 3,649,859 3/1972 v Watt 310/168 3,604,966 9/1971 Liggett.....310/168 3,541,368 11/1970 Jones 310/168 3,646,376 2/1972 Anderson310/171 3,504,208 3/1970 Rivers 310/70 3,486,054 12/1969 Livingston3l0/17l 11 3,739,1i June 12, 1973 Primary ExaminerR. SkudyAttorney-Robert G. Clay [57] ABSTRACT tion. The toothed elements arefurther mounted in magnetic circuit with magnets and a coil'wherebymagnetic flux provided by the magnets follows two separate magneticallyparallel flux paths including the pair of toothed elements and the coil.As the toothed elements are relatively rotated, the net reluctance ofthe flux path through the tooth elements varies and causes a voltage tobe induced in the coil which alternates at a frequency representative ofthe relative speed of rotation of the toothed elements. The other of themagnetically parallel flux paths is of a fixed reluctance and does notextend through the pair of toothed elements.

7 Claims, 4 Drawing Figures Patented June 12, 1973 3,739,211

66 v 76 7 I DIFFERENTIATOR L E i AND INVERTING GENERATOR AMPLIFIER'NVENTOR' ALFRED NMI HASLER TIE J.-

AT-TORNEY 1 MAGNETIC TACHOMETER FIELD'OF THE INVENTION The presentinvention generally relates to tachometers and, more particularly tomagnetic tachometers inherently free of rotationally eccentric errors.

BACKGROUND OF THE INVENTION Tachometers are used to provide indicationsof the angular velocity of rotating equipment, usually motors. One ofthe more common applications of tachometers is in servo systems forcontrolling the speed of rotating equipment. In such applications, thetachometer is em ployed to sense and provide a'signal representative ofthe angular velocity or speed of the rotating equipment. This signal iscompared with a known reference representative of a desired speed and,if the speed of the rotating equipment is other than the desired speed,a corrective error signal is generated. The error signal is coupled tothe rotating equipments drive system to correct the speed of therotating equipment. Precise speed control in such servo systems isdependent upon the tachometer providing a signal that accuratelyrepresents the angular displacement of the rotating equipment.

Typical precision tachometers, both magnetic and optical, include arotatable disc having sensible, distinctive indicies or marks locatedthereon at precise angular intervals. A stationary detector is locatedto detect the marks as the disc is rotated with the rotating equipment.Some tachometers employ a single detector located at one positionrelative to the circumference of the disc and others employ twodetectors located at diametrically opposite positions relative to thecircumference of the disc. Great care must be exercised inmanufacturing, assemblying and aligning these precision tachometers inorder to minimize rotational eccentricity and attendant errors in theprovided speed representative signals. Some of the more common sourcesof rotational eccentricity are misalignment of the axis of the sensiblemarks and the axis of rotation of the disc, and variations in theangular spacing between the sensible marks. While rotationaleccentricity can initially be minimized by the use of carefulmanufacturing, assemblying and aligning techniques in the constructionof the tachometers, such techniques are costly. Furthermore, suchtechniques do not entirely eliminate rotational eccentricity. Subsequentexposures to substantial physical disturbances often causemisalignments, which introduce or increase the rotational eccentricity.

Considerable advantage is therefore to be gained from a tachometerconstruction that is free of errors caused by the presence of rotationaleccentricity. Additional advantages will be realized by simplifying andreducing the cost of construction of precision tachometers.

SUMMARY OF THE INVENTION chometer structure that permits precisiontachometers to be used in environments where they may encounter largephysical disturbances.

A further object of this invention is to provide a tachometer structurethat does not require the precise alignment of its component parts forthe construction of a precision tachometer.

It is still another object of this invention to provide an inexpensivetachometer able to indicate precisely the angular velocity of rotatingequipment.

In accordance with the present invention, two members of magneticallypermeable material are mounted for relative rotation about an axis withfacing portions, which extend circumaxially about the axis, spacedopposed in rotational registration. The members are located to form amagnetic circuit with a magnet and a magnetic flux detector wherebymagnetic flux provided by the magnet follows a path extending betweenthe spaced opposed portions of the members through the regiontherebetween and a path along which the magnetic flux detector islocated. Each member is provided with means for establishing differentreluctances in the flux path between the spaced opposed portions at aplurality of angular intervals about the axis. As the magneticallypermeable members are relatively rotated, the

reluctance of the flux path extending through both of the membersvaries. The varying reluctance causes the flux in the magnetic circuitto vary. This variation in the flux is detected by the magnetic fluxdetector. Since the reluctance will vary according to the rate ofrelative angular displacement of the relatively rotated members, themagnetic flux detector can be arranged to issue signals whosefrequencies are representative of the rate.

The magnetic flux passes between the spaced opposed portions aroundtheir entire rotationally registered locations. As the members arerelatively rotated, the locations of different reluctances associatedwith each of the members pass into and out of registration with those ofthe other member. Each location of different reluctance associated withone member passing into and out of registration with each location ofdifferent reluctance associated with the other member causes a cyclicalflux change in the magnetic circuit. Since this occurs around the entirespaced opposed portions of the relatively rotated members, the sum ofall cyclical flux changes occurring around the entire rotationallyregistered portions represents the cyclical variation of the flux in orreluctance of the flux path of the magnetic circuit.

By establishing a flux path around the entire relatively rotated memberswhose reluctance varies at several locations thereabout as the membersare relatively rotated and summing the flux changes that occur at theseveral locations to obtain a signal representative of the relativeangular displacement of the relatively rotated members, the cyclicalvariation of the flux will accurately represent the relative angulardisplacement of the relatively rotated members. Apparent changes in theangular displacement per unit time characteristic of the previouslydiscussed prior art tachometers as a result of the presence ofrotational eccentricity or varying angular spacing between the sensiblemarks are essentially eliminated by the averaging effect obtained bysumming cyclical flux changes occurring around the entire rotationallyregistered spaced opposed portions. Because the tachometer structure ofthe present invention is inherently less susceptible to. errors causedby the presence of rotational eccentricity, the manufacture, assemblyand alignment of the tachometer are simplified and less expensive.Moreover, since it is less susceptible to such errors, the tachometer isbetter able to withstand large physical disturbances.

BRIEF DESCRIPTION OF DRAWING The foregoing and other advantages andfeatures of the present invention will become more apparent from thefollowing description and claims considered together with theaccompanying drawing of a preferred embodiment of which:

FIG. 1 is a perspective, partially sectioned view of one embodiment ofthe tachometer of the present invention.

FIG. 2 is an axial cross section of the tachometer of FIG. 1 taken alonglines 2-2.

FIG. 3 is a schematic diagram of the tachometer of FIG. 1 showing thepaths of magnetic flux.

FIG. 4 is a block diagram illustrating one use of the tachometer of thepresent invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, thepresent invention is a magnetic tachometer 11 of the type including afirst magnetically permeable member or tachometer wheel 12 mounted forrelative rotation with a second magnetically permeable pickup member 13,which preferably is stationary with respect to the tachometer wheel 12.The tachometer wheel 12 is provided with magnetically sensible indices14 at circumaxially spaced locations. These indices are detected by thetachometer pick-up member 13 as the wheel 12 is rotated to provide anindication of either the angular displacement or rotational velocity ofthe wheel 12. The tachometer wheel 12 is mounted for rotation with ashaft 16 adapted to be coupled to the rotating member of rotatingequipment, such as a capstan or transport motor of a magnetic tapetransport mechanism (not shown).

In accordance with the present invention, the margins of the tachometerwheel 12 and of the pick-up member 13 define rotationally registeredspaced opposed facing portions 17 and 18 extending circumaxially aboutthe axis 19 of rotation of the tachometer wheel 12. In the illustratedembodiment, the facing portions 17 and 18 lie inthe same radial plane ofthe tachometer wheel 12. However, wheel 12 and pick-up member 13 may beconstructed with the circumaxial facing portions in other alignments,such as perpendicular to the radial plane of the wheel 12 or in a planeparallel or at an angle to the radial plane of the wheel 12.

A region 21 is provided between the spaced opposed facing portions 17and 18, preferably, having a higher reluctance than the magneticallypermeable wheel 12 and pick-up member 13. To facilitatethe constructionand operation of the tachometer 11, the magnetically permeable wheel 12and pick-up member 13 are relatively located to define an annular highreluctance air I gap region 21 between the spaced opposed facingportions 17 and 18.

The facing portions 17 and 18 separated by the annular air gap 21 formpart of a magnetic circuit 22 (see FIG. 3) including a source 23 ofmagnetic flux and a magnetic flux detector 24. The elements of thetachometer 11 are arranged to form a magnetic circuit 22 having twopaths for the magnetic'flux provided by the source 23; a fixedreluctance flux path 22a and a variable reluctance flux path 22b. In theillustrated preferred embodiment, the fixed reluctance flux path 22a isformed by the tachometer wheel 12 and the shaft 16 to which it ismounted. The flux detector 24 is located in the fixed reluctance fluxpath 220. The variable reluctance flux path 22b is formed by thetachometer wheel 12, high reluctance air gap 21 and tachometer pick-upmember 13. Means 26 and 27 are associated with each of the tachometermembers 12 and 13, respectively, to establish different reluctances inthe air gap 21 at regular circumaxial intervals about the members. Asthe tachometer wheel 12 and pick-up member 13 are relatively rotated,the different reluctance means 26 and 27 cause the average reluctance ofthe variable reluctance flux path 22b through the facing portions 17 and18 to vary according to the rate of relative angular displacement of thewheel and pick-up member. Because a change in the reluctance of a fluxpath will cause a change in the distribution of the total magnetic fluxprovided by a source, such as occurs between the flux path of themagnetic circuit and the flux path of the leakage flux when thereluctance of the magnetic circuit flux path changes, the variation inthe average reluctance of the variable reluctance flux path 22b will bereflected as a sensible change in the flux in the fixed reluctance fluxpath 22a. This sensible change is detected by the flux detector 24 whichusually is arranged to responsively generate a signal representative ofthe rate of relative angular displacement of the wheel 12 and pick-upmember 13. It is the means 26 associated with the tachometer wheel ormember 12 to establish different reluctances in the variable reluctanceflux path 2212 that serves as the magnetic sensible indices 14 detectedby the pick-up member 13. And it is the means 27 associated with thepick-up member 13 that serves to detect the sensible indices 14 of thetachometer wheel 12.

A more comprehensive understanding of the tachometer 11 of the presentinvention can be had upon a detailed consideration of the particularconstruction of the one embodiment of the tachometer 11 illustrated byFIGS. 1 and 2. More particularly, the tachometer wheel 12 is a serrateddisc 25 of soft iron having a plurality of outwardly projectingserrations or teeth 28 equally spaced about its circumference andseparated by root segments 29. The alternating teeth 28 and rootsegments 29 are the means 26 that establish different reluctances aboutthe disc 25 and define the sensible indices 14. The serrated disc 25' isfastened proximate its center by screws 31 to a rigid coupler 32 pressfit to the shaft 16 to rotate therewith. The coupler 32 and shaft 16 maybe arranged to form part of the fixed reluctance flux path 22a and, insuch event, would be constructed from a material having a magneticpermeability greater than air. However, as long as a flux leakage pathof air is provided between the disc 25 and the flux detector 24,sufficient flux will be coupled to the flux detector 24 to enable thedetection of flux changes in the flux path 22a.

The shaft 16 is supported for rotation at its ends by journal bearings33 and 34 seated within the tachometer housing 36. The tachometerhousing 36 includes a cylindrical section 37 having one integral endplate 38 and one demountable end plate 39. The demountable end plate 39is secured by bolts 41 to a flange 42 extending outwardly from the sideof the cylindrical section 37. To house and hold the bearings in place,the

end plates 38 and 39 are provided with cylindrical extensions 43 and 44respectively extending in the direction of the axis 19 outward of thehousing 36 to define receptacles 46 and 47 for receiving the journalbearings 33 and 34. In the receptacle 46, the journal bearing 33 isseated on a shoulder 48 at the bottom of the receptacle and held thereinby a lockscrew 49 threadingly engaging the open end of the cylindricalextension 43. The lockscrew 49 is aperatured at its center to permit theshaft 16 to extend therethrough and be coupled to the rotatingequipment. In the receptacle 47, the journal bearing 34 is seated on theshoulder 51 at the bottom of the receptacle and held therein by alockscrew 52 threadingly engaging the open end of the cylindricalextension 44. An end trust bearing 50 is interposed the end of the shaft16 and the lockscrew 52.

Surrounding the serrated .disc 25 is a stationary annular member 53 ofsoft iron having a serrated surface 18 facing the serrated surface 17 ofthe disc 25. The serrated annular member 53 defines a plurality ofprojecting serrations or teeth 54 equally spaced about its innercircumference and separated by root segments 56. To keep the reluctanceof the space 21 separating the teeth 28 and 54 within limits whichpermit substantial flux to follow path 22b, the inner radius of theannular member 53 defined by the tips of the teeth 54 is made aboutfive-thousandths of an inch greater than the radius of the disc 25defined by the tips of the teeth 28. The alternating teeth 54 and rootsegments 56 form the means 27 for detecting the sensible indices 14, oralternating teeth 28 and root segments 29, of the rotatable disc 25. Thenumber of teeth 54 and their circumferential spacing are selectedrelative to the number and spacing of teeth 28 of the disc 25 so thatall of the teeth 28 simultaneously move into and out of registrationwith teeth 54 of the annular member 53. Preferrably, the disc 25 and theannular member 53 have the same number of teeth 28 and teeth 54,respectively. Furthermore, the circumaxial width of the teeth 28 attheir tips is selected to be less than the corresponding circumaxialwidth of the teeth 54 to facilitate detecting the alternating teeth 28and root segments 29 of the rotable disc 25. In one embodimentconstructed, the disc 25 had an outside diameter of about 5 34 inchesand 384 teeth were provided on both the disc 25 and annular member 53,with the teeth 28 of the disc 25 having a circumaxial tip width often-thousandths of an inch and the teeth 54 of the annular member 53 acircumaxial tip width of twelve-thousands of an inch. It should beappreciated that the tachometer 11 of the present invention could alsobe constructed-with the disc 25 stationary and annular member 53 rotablymounted. in such an embodiment, the alternating teeth 54 and rootsegments 56 of the rotably mounted annular member 53 would serve as thesensible indices 14 and the alternating teeth 28 and root segments 29 ofthe stationary disc 25 would serve as the means for detecting thesensible indices.

The annular member 53 is supported in the same radial plane as the disc25 by the integral cylindrical member 57 coaxial with the cylindricalhousing section 37 and secured as by welding to the disc shaped supportplate 58. The support plate 58 is held between the demountable end plate39 and the housing flange 42 and is provided with a centrally locatedaperture 59 which permits the shaft 16 to extend to the bearing 34.Enlarged bolt apertures are provided in the support plate 58 to permitit to be moved radially so that the annular member 53 and disc 25 can bealigned about the axis 19. To adjust the relativeradial positions of thedisc 25 and annular member 53 for optimum output signal and so that theyare maintained out of contact; four set screws 60 are located at 90intervals about the cylindrical section 37 to extend through its walland bear against the cylindrical member 57. The set screws 60 areadjusted prior to tightening the bolts 41 to center the disc 25 andannular member 53. Once centered for optimum signal output, the boltsare tightened to hold the support plate 58, hence, annular member 53,

in place relative to the disc 25.

The support plate 58 also supports the magnetic flux source 23 andmagnetic flux detector 24. More particularly, the detector 24 is aninduction coil 61 wound on a coil form 62. The coil form 62 is supportedby the plate 58 to surround the shaft 16 between the plate 58 and theserrated disc 25. Screws 63 are provided to fasten the coil form 62 tothe plate 58. A pair of leads 64 extend through apertures provided inthe'cylindrical stem 57 and housing section 37 to couple the inductioncoil 61 to a signal processing system 66 (see FIG. 4).

To support the magnetic flux source 23 relative to the toothed elements25 and 53, a circular plate 67 is fastened by screws 68 to the end ofthe coil form 62 proximate the serrated disc 25. The circular plate 67extends radially beyond the perimeter of the coil form 62 and isapertured at its center to permit the shaft 16 to pass to the bearing34. Three permanent magnets 23a, 23b and 230 are located at 120intervals about a circle surrounding the coil form 62 on the side of theplate 67 facing the coil form. Screws 69 are employed to fasten thepermanent magnets 23a, b and c to the circular plate 67. Because theplate 67 is spaced from the serrated disc 25 to provide clearance forthe coupler 32 and the heads of the screws 31, a magnetically permeablering 71 is provided between the circular plate 67 and serrated disc 25to reduce the reluctance of the flux path from the magnets to theserrated disc. The ring 71 is constructed to have a mean radius equal tothe radius of the circle about which the permanent magnets 23a, b and care located and is fastened to the circular plate 67 by screws 72 tooverlay the circularly disposed magnets. To allow the serrated disc 25to rotate freely relative to the stationary ring 71, the ring 71 has anaxial thickness which is a few thousandths of an inch less than thedistance separating the confronting surfaces of the serrated disc 25 andcircular plate 67.

When employed to provide an indication of the speed of rotatingequipment such as a motor, the shaft 16 is linked to rotate with therotating member of the motor. As the motor is driven, the disc 25rotates with the shaft 16 whereby its teeth 28 alternately pass into andout of registration with the teeth 54 of the stationary annular member53 at a rate corresponding to the speed of the rotating member. Thispassing of the teeth 28 and 54 into and out of registration causes thereluctance of the path 22b to vary cyclically at the rate at which theteeth pass into and out of registration. As discussed hereinabove, achange in the reluctance of the path 22b causes a corresponding changein the distribution of the magnetic flux in the paths 22a and 22b.Therefore, the varying reluctance of the path 22b causes a correspondingcyclical variation in the flux in the path 22a. The cyclically varyingflux in path 22a induces a corresponding varying voltage signal in theinduction coil 61.

7 However, the induction coil could be located in the path 22b to detectdirectly the variation of the reluctance by sensing the changing flux inthat path. In any event,'the induced voltage signal varies at afrequency corresponding to the rate at which the teeth 28 and 54 passinto and out of registration. For example, a tachometer 11 having 384teeth on both the disc and annular member 53 generates a voltage signalhaving a frequency equal to the product of 384 and the number ofrevolutions of disc 25 per second.

The cyclically varying voltage signal induced in the coil 61 is coupledby leads 64 to the signal processing system 66, which operates tocondition the signal for use in speed determining or controllingapparatus. The signal processing system 66 illustrated in FIG. 4 is acommon system employed in speed control apparatus for converting acyclically varying speed representative voltage signal to a train ofpulses having a pulse repetition rate proportional to the speed of theequipment being controlled. Such systems include a preamplifier 73 whoseinput is coupled to receive the varying voltage signal present on leads64. The output of the preamplifier 73 extends to the input of a limiter74, which is operated as a squaring circuit. The limiter 74 converts thecyclically varying voltage signal to a square waveform with well definedsignal state transitions of short durations. A differentiator andinverting amplifier circuit 76 is coupled to receive the square waveformoutput from the limiter 74 and operates to generate a train ofunidirectional trigger pulses, one for each signal state transition ofthe received square waveform. These trigger pulses are coupled toactuate a pulse generator 77 to issue accurately spaced pulses each of aprecise duration. The pulse generator 77 is provided with an outputterminal 78 for coupling the speed control apparatus to receive thespeed representative pulses.

What is claimed is:

1. A magnetic tachometer comprising a first member of magneticallypermeable material defining a surface extending circumaxially about anaxis; a second member of magnetically permeable material positionedrelative to the first member defining a surface extending circumaxiallyabout said axis; means for supporting the first and second members forrelative rotation about said axis with their said extending surfacesspaced opposed in rotational registration; means for varying thereluctance in a first flux path extending through the members .andbetween the spaced opposed surfaces at a plurality of angular intervalsabout the axis as the members are relatively rotated; means forproviding a second flux path of fixed reluctance separate from andmagnetically parallel to the first flux path; a source of magnetic fluxsupported relative to the members and magnetically parallel flux pathsto establish magnetic flux in the first flux path and in the secondseparate flux path; and a magnetic flux detector positioned to detectmagnetic flux in one of the separate magnetically parallel paths.

2. The magnetic tachometer according to claim 1 wherein the magneticflux detector is positioned to detect flux in the second flux path offixed reluctance.

3. The magnetic tachometer according to claim. 1

wherein the means for supporting the members for relative rotationincludes a shaft of magnetically permeable material mounted for rotationabout the axis and one of said members is fastened to the shaft forrotation therewith and is magnetically coupled to the shaft to form partof the second flux path of fixed reluctance.

4. The magnetic tachometer according to claim 3 wherein the magneticflux detector includes an induction coil supported in a stationaryposition surrounding the shaft to permit said shaft to rotate relativeto said coil.

5. The magnetic tachometer according to claim 4 wherein the first one ofthe members is a disc fastened at its center to the shaft for rotationabout the axis, the disc is provided with serrations forming alternateprojective and root segments along its surface extending circumaxiallyabout the axis for varying the spacing between the members therebyestablishing different reluctances, the second member includes anannular portion supported in a stationary position, the annular portionhaving serrations forming alternate projective and root segments alongits surface extending circumaxially about the axis for varying thespacing between the members thereby establishing different reluctances,and the source of magnetic flux includes a permanent magnet supported ina stationary position at one side of the plane of the disc to couplemagnetic flux from one of its poles to the disc while permitting thedisc to rotate relative to the magnet.

6. The magnetic tachometer according to claim 5 wherein the disc has anaxially extending surface about its circumferential perimeter definingits circumaxially extending surface, the serrations about the disc arelocated'along the axially extending surface to form alternate projectiveand root segments about the axis with the projective segments spaced atregular angular intervals about the axis, the annular member is acylindrical structure journally supported at its opposite ends to theshaft to permit the shaft to rotate relative thereto, the cylindricalstructure is supported to surround the disc with a space therebetween',the cylindrical structure has a radial inwardly extending portion havingan axially extending surface defining the circumaxially extendingsurface of the annular member, the serrations about the annular memberare located along its axially extending surface to form alternateprojective and root segments about the axis with the projective segmentsspaced the regular angular intervals about the axis, and the permanentmagnet supported with its poles spaced from the disc and the cylinderand the shaft to form air gaps in the magnetic flux paths at its poles.

7. The magnetic tachometer according to claim 5 wherein the disc and theannular portion have the same number of alternate projective and rootsegments with the projective segments spaced the same regular angularintervals about the axis, and each of the projective segments has a tipof a finite width in the circumaxial direction with said tip width ofeach projective segment of one magnetically permeable member the same,the circumaxial width of the tips of the projective segments of the discis smaller than the circumaxial width of the tips of the projectivesegments of the annular portion. l k

1. A magnetic tachometer comprising a first member of magneticallypermeable material defining a surface extending circumaxially about anaxis; a second member of magnetically permeable material positionedrelative to the first member defining a surface extending circumaxiallyabout said axis; means for supporting the first and second members forrelative rotation about said axis with their said extending surfacesspaced opposed in rotational registration; means for varying thereluctance in a first flux path extending through the members andbetween the spaced opposed surfaces at a plurality of angular intervalsabout the axis as the members are relatively rotated; means forproviding a second flux path of fixed reluctance separate from andmagnetically parallel to the first flux path; a source of magnetic fluxsupported relative to the members and magnetically parallel flux pathsto establish magnetic flux in the first flux path and in the secondseparate flux path; and a magnetic flux detector positioned to detectmagnetic flux in one of the separate magnetically parallel paths.
 2. Themagnetic tachometer according to claim 1 wherein the magnetic fluxdetector is positioned to detect flux in the second flux path of fixedreluctance.
 3. The magnetic tachometer according to claim 1 wherein themeans for supporting the members for relative rotation includes a shaftof magnetically permeable material mounted for rotation about the axisand one of said members is fastened to the shaft for rotation therewithand is magnetically coupled to the shaft to form part of the second fluxpath of fixed reluctance.
 4. The magnetic tachometer according to claim3 wherein the magnetic flux detector includes an induction coilsupported in a stationary position surrounding the shaft to permit saidshaft to rotate relative to said coil.
 5. The magnetic tachometeraccording to claim 4 wherein the first one of the members is a discfastened at its center to the shaft for rotation about the axis, thedisc is provided with serrations forming alternate projective and rootsegments along its surface extending circumaxially about the axis forvarying the spacing between the members thereby establishing differentreluctances, the second member includes an annular portion supported ina stationary position, the annular portion having serrations formingalternate projective and root segments along its surface extendingcircumaxially about the axis for varying the spacing between the membersthereby establishing different reluctances, and the source of magneticflux includes a permanent magnet supported in a stationary position atone side of the plane of the disc to couple magnetic flux from one ofits poles to the disc while permitting the disc to rotate relative tothe magnet.
 6. The magnetic tachometer according to claim 5 wherein thedisc has an axially extending surface about its circumferentialperimeter defining its circumaxially extending surface, the serrationsabout the disc are located along the axially extending surface to formalternate projective and root segments about the axis with theprojective segments spaced at regular angular intervals about the axis,the annular member is a cylindrical structure journally supported at itsopposite ends to the shaft to permit the shaft to rotate relativethereto, the cylindrical structure is supported to surround the discwith a space therebetween, the cylindrical structure has a radialinwardly extending portion having an axially extending surface definingthe circumaxially extending surface of the annular member, theserrations about the annular member are located along its axiallyextending surface to form alternate projective and root segments aboutthe axis with the projective segments spaced the regular angularintervals about the axis, and the permanent magnet supported with itspoles spaced from the disc and the cylinder and the shaft to form airgaps in the magnetic flux paths at its poles.
 7. The magnetic tachometeraccording to claim 5 wherein the disc and the annular portion have thesame number of alternate projective and root segments with theprojective segments spaced the same regular angular intervals about theaxis, and each of the projective segments has a tip of a finite width inthe circumaxial direction with said tip width of each projective segmentof one magnetically permeable member the same, the circumaxial width ofthe tips of the projective segments of the disc is smaller than thecircumaxial width of the tips of the projective segments of the annularportion.